Crystalline thermoplastic polyester resins such as polyalkylene terephthalate resins have been used extensively as an engineering plastic in various fields owing to their excellent mechanical and electrical properties, as well as their physical and chemical characteristics and good processability. Thus, thermoplastic polyester resins have been used to form molded parts including automotive, electrical and electronic applications.
Diversifying the end-use applications for thermoplastic polyester resins, however, often requires specific performance and/or property characteristics. For example, one problem which has prevented thermoplastic resins from being used in a greater number of end-use applications is the propensity for thermal decomposition products (e.g. terephthalic acid) generated during molding to "bleed" from the resin. These decomposition products, which are solid at normal temperatures are thus deposited on the molding apparatus resulting in the deterioration of the surface appearance of a resulting molded article. The present increased demand for a polyester resin and the greater range of its end-use applications make the problem associated with surface appearance increasingly more important from a material selection point of view. Undesirable surface defects may therefore preclude thermoplastic polyester resins from being employed in specific areas of use, especially films and sheets.
For example, when an ordinary polyethylene terephthalate (PET) or polybutylene terephthalate (PBT) is melt-molded into a film in an oxygen atmosphere by, e.g., a process which comprises melt-extruding the resin through a slit and then cooling the extrudate with a chilled roll or a guide bar to give a solid film, solid decomposition products will gradually deposit on the various components in contact with the resin, for example, the slit die, chilled roll and/or guide. These deposits roughen the surface of the formed film, thereby lowering the film's surface gloss. In order to avoid lower film surface gloss, the die, chilled roll and/or guide bar must be cleaned frequently--a complicated operation resulting in production down time and a concomitant economical losses.
Low molecular weight additives, such as various antioxidants or catalyst deactivators, have conventionally been added to thermoplastic polyester resins in order to solve the thermal decomposition problems mentioned above. However, polyester resins containing such an additive will, in many cases, exhibit other problems, such as the bleedout of the additive itself and/or a decrease in the surface gloss of molded articles due to the presence of the additive, even though the heat stability of the resin is improved. Therefore, the development of a more effective means for solving the problems noted above has been needed. It is towards fulfilling such a need that the present invention is directed.
It has now been found that a thermally stable, high-quality aromatic copolyester having a low carboxyl end group (CEG) concentration which does not experience detrimental bleedout of thermal decomposition products (e.g., terephthalic acid) during melt-molding in an oxygen atmosphere (such as melt extrusion) can unexpectedly be obtained by the introduction of a specified comonomer to be described in greater detail below. The resin compositions can be molded by, e.g. extrusion, into films or sheets having good surface appearances without depositing decomposition products which are normally solid at ordinary temperatures (e.g., about 20.degree. C.) during molding.
The present invention relates more specifically to a polyester resin having a CEG of no greater than 40 (meq/kg) and high melt-phase heat stability to an extent that substantially no thermal decomposition products (such as terephthalic acid) are formed during melt-molding. The novel polyester resins of this invention are produced mainly from (a) an aromatic dicarboxylic acid or its ester-forming derivative, and (b) an aliphatic dihydroxy compound having 2 to 8 carbon atoms, which are copolymerized with (c) an ester-forming comonomer compound of the following general formula (I): EQU HO--R--O--A--O--OH (I)
wherein A represents a divalent organic radical having at least one aromatic ring, R represents a divalent organic radical selected from the group consisting of aliphatic hydrocarbon groups and polyoxyalkylene groups each having 2 to 8 carbon atoms.
The ester-forming comonomer is employed in an amount sufficient to achieve between about 0.01 to about 7 molar percent (based on the aromatic dicarboxylic acid) of the ester-forming comonomer in the resulting copolyester chain.
The concentration of the carboxyl end group of the copolyesters according to this invention is expressed hereinafter in milliequivalents per kilogram (meq/kg) and as determined by dissolving a weighed amount of a polyester in mixture of benzyl alcohol and chloroform and titrating with benzyl alcoholic sodium hydroxide.
Further aspects and advantages of the present invention will become evident after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follow.